Fix bug #62535 - $_SESSION[$key]["cancel_upload"] doesn't work as documented

When uploading multiple files, the array in the session where the progress
data is stored (and cancel_upload key is set) was overwritten whenever the
progress data was updated, so pending uploads would proceed anyway.
Merge branch 'PHP-5.4' into PHP-5.5

* PHP-5.4:
  Fix bug #62535 - $_SESSION[$key]["cancel_upload"] doesn't work as documented